在Java 8中对比较器的误解 [英] Misunderstanding about Comparator in java 8

问题描述

public class Test {
    public static void main(String[] args) {
        List<Pair<String, Integer>> list = new ArrayList<>();
        list.add(new Pair<>("1", 8));
        list.add(new Pair<>("3", 2));
        list.add(new Pair<>("2", 15));

        list.stream()
            .sorted(Comparator.comparingInt(p -> p.v))
            .map(p -> p.k)
            .forEach(System.out::println);

    }
}

class Pair<K, V> {
    K k;
    V v;
    public Pair(K k, V v) {
        this.k = k;
        this.v = v;
    }
}

好的，你知道这段代码正在打印我的一对从最高值关联到最高值的键，所以我得到预期的输出：

Ok, as you understood this code is printing my pair keys from the lowest value associated to the highest so I get the expected output:

3
1
2

3 1 2

到目前为止一切顺利。现在我想做相反的事情，我想我只需要这样做

So far so good. Now I wanted to do the reverse, I thought I would only have to do

list.stream()
    .sorted(Comparator.comparingInt(p -> p.v).reversed())
    .map(p -> p.k)
    .forEach(System.out::println);

但我收到编译错误：

v cannot be resolved or is not a field

所以它似乎 comparisonInt 返回 Comparator< Object> 。为什么会这样？它不应该返回 Comparator< Integer> ？

So it seems like comparingInt is returning a Comparator<Object>. Why it so? Shouldn't it return a Comparator<Integer> ?

这些都可以使用Eclipse Luna版本1和javac重现。

These are both reproductible with Eclipse Luna version 1 and javac.

javac -version => 1.8.0
java  -version => java version "1.8.0_25"

哦也随意更改我的问题的标题你找到了它太笼统了，但我找不到正确的术语

Oh also feel free to change the title of my question is you find it too general, but I couldn't find the correct terms

推荐答案

我认为这种类型推断是失败的，基本上 - 因为 reverse（）调用会妨碍预期的参数类型 sorted（）和lambda表达式。

I believe it's just that type inference is failing, basically - because the reverse() call gets in the way of the expected argument type of sorted() and the lambda expression.

如果您明确指定类型为 comparisonInt ，则可以这样做：

You can do it if you specify the type to comparingInt explicitly:

list.stream()
    .sorted(Comparator.<Pair<String, Integer>>comparingInt(p -> p.v).reversed())
    .map(p -> p.k)
    .forEach(System.out::println);

或者如果您只是先声明比较器：

Or if you just declare the comparer first:

Comparator<Pair<String, Integer>> forward = Comparator.comparingInt(p -> p.v);
list.stream()
    .sorted(forward.reversed())
    .map(p -> p.k)
    .forEach(System.out::println);

我觉得应该有一个 Stream.reverseSorted 所以让这类事情变得非常简单，但它看起来并不存在：（

It feels to me like there should be a Stream.reverseSorted so make this sort of thing really easy, but it doesn't look like it exists :(

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